1. Field of the Invention
The present invention relates to polymeric compositions particularly well suited for agricultural purposes. More particularly, the present invention relates to polymeric films or sheets formulated for use as a cover material for growing plants or crops.
2. Description of the Prior Art
Conventional polymeric film such as polyethylene, copolymers of ethylene, and other polyolefins have been extensively used as cover materials for protecting crops in greenhouses and the like. Although the conventional materials have certain advantages such as low cost, light weight, etc. they also possess certain operational disadvantages which can be summarized as follows:
(A) They are permeable to long wave infra-red radiation particularly within the range of between approximately 7 and 14 microns (7.multidot.10.sup.-6 and 14.multidot.10.sup.-6 m) which is precisely the area of greatest transmission of the dry atmosphere. Consequently, the covers or greenhouses covered with these films lose considerable heat through radiation, particularly on clear nights when the earth and the plants act as a low temperature black body transmitter causing the phenomenon known as thermic inversion to occur. This phenomena can occur when the temperature inside the cover or greenhouse is only a few degrees lower than the temperature on the outside thereby producing frost and/or increasing the cost of the necessary energy when the cover or greenhouse is heated artificially.
(B) These films are poor light diffusers creating areas of deep shadow inside the covers or greenhouses produced by the structural support and the plants themselves.
(C) These films are permeable to ultra-violet radiation and therefore provide no protection from the harmful effect of this radiation on the plants.
In an effort to minimize certain of the aforementioned disadvantages the art has suggested the incorporation of additives such as silica, aluminum silicate or aluminum hydroxide. The following patents are noted which relate to the subject matter under consideration and of which applicant is aware:
______________________________________ Patent No. Date ______________________________________ French 1,574,088 April 16, 1968 French 2,213,954 January 15, 1973 (Patent of addition to French 1,574,088) Japanese 7213.853 May 15, 1968 Belgium 845,925 September 9, 1975 Japanese 5-1074-846 December 18, 1974 Japanese 5-0088-147 November 8, 1973 ______________________________________
By adding to polyolefin film products such as silica, aluminum silicate or aluminum hydroxide, transmittance in the area of the spectrum included between 7 and 14 microns is considerably reduced and the strength of light diffusion is increased, however, such formulations have resulted in the following drawbacks:
(A) Silica and aluminum silicates are generally very abrasive causing serious wear problems in the equipment used to formulate the composition and the later manufacture of the film.
(b) Silica and aluminum silicates have the effect of accelerating the degradation of these films.
(c) Although aluminum hydroxide presents none of the above problems, it only has an absorbing effect in wavelengths greater than 10 microns (10..sup.-6 m). This means that it is only really effective when mixed with silica or aluminum silicate and thus the above problems cannot be avoided.
It is also known that ethylene and vinyl acetate copolymer films (EVA copolymer) increase their absorption in the area of the spectrum included between 7 and 14 microns as the vinyl acetate content is increased but its light diffusion effect is also very low irrespective of vinyl acetate content.